I took a look at it. It’s a library for Arduino. However it is theoretically possible to use it as part of my firmware but it needs some C++ in Embedded Systems programming experience in doing it since you need to use interrupts refactor the code a little bit and add a new Task and some other config stuff in order to integrate it properly. But I will not be able to do that in the next days/weeks since I have a lot of other stuff to do. If you need it urgent maybe someone else here in the forum with C++ knowledge can do it for you. If I did it I would need the sensor too in order to be able to test it properly.
Sure there’s also a possibility to quick and dirty hack it into the firmware. But from my experience in the long term it often takes as much or even more time than doing it right at the first time. But If you know somebody with C++ knowlegde and embedded systems he should be able to do it for you.
Or if you only need to use this Flowmeter I recommend you to check the forum here for arduino Lora Firmware. I’m pretty sure there is an Arduino project somewhere that uses Lora. Then you would only need to add the flowmeter stuff. Arduino has a big beginner friendly community its likely that you’ll find people here or in the internet who can help you if you struggle.
Edit: maybe worth a try: How to build your first TTN node: Arduino + RN2483